This invention relates to weighing apparatus employing a variable capacitance load cell and more particularly to an arrangement for automatically verifying the correct operation of such weighing apparatus.
Weighing scales, particularly those intended for commercial use, must meet a number of requirements for performance while not being excessively costly. The scales must be sufficiently accurate to satisfaction their application requirements and, in many cases, to satisfy public weights and measures authorities. The required accuracy must also be maintained as long as the scale is in use. Many weighing scales in use in recent years employ strain gauge load cells as the weight sensing element and process a weight signal through analog and digital circuits before the weight indication in digital form is displayed or otherwise utilized. Relatively slight changes in the parameters in the signal path caused by, for example, aging or failure of a component, can produce unacceptable error in the weight indication.
It is known to provide automatic verification procedures in electronic scales to periodically verify the accuracy of the scale. In U.S. Pat. No. 4,310,893, for example, there is disclosed an automatic verification procedure in a weighing scale employing a strain gauge load cell. In that scale, an analog signal of sufficient accuracy is introduced into the electrical system as a weight simulating signal after each weighing operation has been completed, and no load is on the scale. This verification signal is then processed by the entire analog and digital circuits of the scale and compared with a previously stored reference digital value corresponding to correct operation of the scale in response to the verification signal. If the compared values correspond within predetermined tolerance limits, the scale is in condition for further weighing operations. If the difference is not within tolerance limits, the scale may be disenabled from further weighing.
Recently, there have appeared scales intended for applications requiring high accuracy and employing variable capacitance load cells as the load sensing element. These scales also employ analog and digital circuits for obtaining and processing a weight signal, and would benefit from an automatic verification procedure.